module T::Private::Abstract::Data

insert `key` with a value of `default` and return `default`.
Works like `setdefault` in Python. If key has already been set, return its value. If not,

def self.set_default(mod, key, default)
  if self.key?(mod, key)
    self.get(mod, key)
  else
    self.set(mod, key, default)
    default
  end
end